1. Field of the Invention
The invention relates to a method of reading a recording medium and to a system applying this method.
The invention can be applied notably to the reading of magnetic or optical recordings and, in this context, to the reading of high-density recording. It can be applied preferably in recording systems such as computer peripherals or any professional systems.
It can be extended to recordings on optical tape and on magnetic or optical disks when it is sought to read several adjacent tracks therein in parallel.
2. Description of the Related Art
High-density recording on parallel tracks raises a twofold problem in re-reading: track following and track separation. The small width of the tracks (which is smaller than 20 .mu.m) means that it is difficult, on a tape reader, to ensure the precision of the track-following operation on the basis solely of the mechanical guidance of the edge of the tape.
The need to ensure the inter-operational quality of the tapes and readers aggravates this difficulty.
Furthermore, to obtain a good signal-to-noise ratio in reading, it is necessary to re-read the entire track width. This rules out the existence of a barrier between tracks and induces phenomena of cross-talk in reading from track to track.
The present systems of high-density recording on tape are based on the use of an analyzer rotating drum. They require the implementation of track-following and cross-talk reduction techniques as explained here below.
The reading heads are generally wider than the tracks that they read and their gap has an azimuth angle (6-10-20) with respect to the normal in the direction of run. This azimuth is inverted from one head to another and, hence, from one track to the adjacent track so that, at reading, the cross-talk due to the neighboring tracks is attenuated all the more so as the frequencies are high. The cross-talk therefore is exerted only on the low part of the spectrum.
In analog video systems, there are color difference signals of low spatial resolution and it is seen to it that the cross-talk affects (mixes) only the identical colors of neighboring lines.
When the information element recorded is digital, steps are taken to increase the azimuth angle. For example, 10.degree. for the 8 mm PCM system and 20.degree. for the digital recording audio system (DAT). The system uses a channel code with little energy in the low frequencies (for example code 8-10 of the DAT).
The particular kinetics of rotating head systems having tracks that are inclined with respect to the edge of the tape has the consequence wherein the track-following operation can be done by making the rotational phase of the drum vary, the tape speed being constant. However, it is necessary to have an error signal available on the basis of information elements written on the tape.